Atomic force microscopy (AFM) has been extensively used to map the physical topography of samples on a sub micron or even nanometer scale. In AFM, a nanometer-scale probe tip descends from a free end of a microcantilever. The tip is dragged or tapped along the surface of a sample and the deflection of the cantilever is measured to thereby determine the topography of the surface. Due to the simple structure of the tip, an AFM can measure only topographic properties of the sample, that is, its physical profile. Akamine et al. in seminal U.S. Pat. No. 4,943,719 has disclosed many ways to form an AFM probe.
Microwave impedance microscopy has been developed based on many concepts of AFM in order to measure the electrical properties of sample on similar microscopic scales, as described by Kelly et al. U.S. Pat. No. 6,825,645. Microwave signals generally encompass electrical signals in the frequency range of 100 MHz to 100 GHz but much equipment operates in the range of 1 to 10 GHz. In microwave impedance microscopy, a metal tip is supported on a microcantilever and descends from its free, distal end. Microwave probe signals are impressed on the metal probe tip and microwave electronics measure signals thereby emitted from the sample to determine the microwave impedance of the sample. The impedance may include both real and imaginary parts, e.g., conductivity and dielectric constant, as described by Kelly et al. in U.S. Pat. No. 7,190,175. Improved versions of the microwave probe tip and its cantilever are described by Lai et al. in US patent application publication 2010/0218286.
The design of the microwave probe is inherently more complicated than the relatively simple structure of an AFM probe for a number of reasons. The probe tip should be composed of a metal and the metal tip must be electrically connected to a wire bond pad on or near the fixed, proximal end of the cantilever in order to apply the microwave signal to the probe tip to effect near-field microwave microscopy. Similar to an AFM probe tip, the apex of the probe tip must be sharp to ensure fine spatial resolution. Such sharpness is often achieved by anisotropically etching crystalline silicon in an AFM probe, but metals needed for microwave microscopy cannot be similarly anisotropically etched. An electrically shielded structure is needed to assure that only the probe tip interacts with the sample to thereby suppress noise. The conducting path resistance and the capacitance to ground from the conducting path along the cantilever must be small enough to obtain strong microwave signals both for the probe signal and for the reflected signal. An AFM probe tip has no similar needs for electrical shielding and low impedance of the conducting path. The cantilever advantageously should be relatively straight to assure that the tip contacts the sample. Also advantageously, the cantilever should not bend with changing temperatures. Such temperature independence enables microwave impedance microscopy to be applied over vast temperature ranges, especially for low temperatures.
Due to these problems and the necessary complexity of a microwave probe structure, the fabrication of microwave probes has proven to be difficult. The most common design of the near-field microwave probe includes an etched metal tip, but metal etching limits the spatial resolution to no less than several micrometers. Another approach fabricates sharper microwave probe tips by a focused-ion beam (FIB). In this method, a silicon nitride microcantilever is fabricated to include shielded metal traces (conduction paths) and a platinum tip is deposited on the cantilever by FIB. Although diameters of FIB tips can be as small as 200 nm, such relatively large tips are too large for many applications in nano science and the characterization of nano materials. Furthermore, the process of fabricating FIB-deposited tips is expensive and time consuming and this type of probe tip cannot be batch fabricated, that is, many tips simultaneously developed on a wafer and thereafter separated for use.
Accordingly, the need has developed for a method to batch fabricate microwave probes with sharp tip apexes, metal shields along the cantilever and adjacent the probe tip, and a structure optimizing resistance and capacitance and a balanced structure for improved mechanical and thermal properties.